A piezoelectric actuator comprises a piezoelectric element in which a piezoelectric film having an electromechanical transducing function is interposed between two electrodes, the piezoelectric film being constituted by a crystallized piezoelectric ceramic. A compound oxide having a perovskite crystalline structure which can be expressed by the chemical formula ABO3 is known as such a piezoelectric ceramic. As an example, lead zirconate titanate (PZT), in which lead (Pb) is applied to the zirconate titanate (PZT), in which lead (Pb) is applied to the A and a mixture of zirconium (Zr) and titanium (Ti) is applied to the B, is known.
Conventionally, Pt has been used as the electrode material of a PZT-type piezoelectric element. Pt has a face-centered cubic lattice (FCC) structure, which is a most densely-packed structure, and hence has a strong self-orientation property such that when deposited on an amorphous material such as SiO2, the Pt is strongly oriented in (111) orientation and the orientation quality of the piezoelectric film thereabove improves. However, there is a problem in that due to the strong orientation property, columnar crystals grow, causing Pb and so on to become likely to diffuse along the grain boundary of the lower base. A problem also occurs in relation to the adhesiveness of Pt and SiO2.
When Ti is used to improve the adhesiveness between Pt and SiO2, and TiN or the like is used as a barrier layer to prevent the diffusion of Pb and the like, the electrode structure becomes complicated, causing oxidation of the Ti, diffusion of the Ti into the Pt, and a resultant deterioration in the crystallinity of the PZT. As a result, electrical characteristics such as the piezoelectric characteristic deteriorate.
Since such problems exist in Pt electrodes, research is being conducted in the fields of ferroelectric memory and so on into the use of conductive oxide electrode materials such as RuOx or IrO2. Among these materials, strontium ruthenate has the same perovskite crystalline structure as PZT, and hence has an excellent bonding property at the interface, facilitates the epitaxial growth of PZT, and also exhibits an excellent characteristic as a Pb diffusion barrier layer.
However, when strontium ruthenate is used as the bottom electrode of a piezoelectric element, the substance which constitutes the layer therebelow must satisfy a physical property as a diaphragm and a chemical property for appropriately controlling the orientation of the strontium ruthenate and ensuring adhesiveness with the substrate and bottom electrode.